1. Field of the Invention
The invention relates to a compact, man-portable, infrared camera that is usable in an operational military environment by military personnel. The camera uses two-color radiometric techniques to determine the emissivity and temperature images displaying the spatial distribution of temperature and emissivity for extended objects. The process includes corrections for the atmospheric path through which the object is being viewed and for the environmental radiance being reflected off the object. The camera is designed for the decoupling of the reflected and emitted radiation from the object and the direct solution for the emissivity.
2. Background of the Invention
All surfaces emit thermal radiation. However, at any given temperature and wavelength, there is a maximum amount of radiation that any surface can emit. If a surface emits the maximum amount, it is known as a blackbody. A blackbody has an emissivity of 1.0 at all temperatures and wavelengths. Most surfaces are not blackbody emitters, and emit some fraction of the amount of thermal radiation that a blackbody would. Emissivity is the ratio of radiation emitted by a surface and the theoretical radiation predicted by Planck's law.
The problem to be solved is to develop a robust, hand-held infrared measurement device to evaluate the infrared characteristics of an object of interest, such as aircraft and other vehicles. There are currently available laboratory instruments capable of measuring emissivity. These instruments are large, difficult to operate, and require careful control of the laboratory environment. As such, they are unsuitable for robust, simple, and man-portable field operations. There are also satellite instruments that attempt to measure emissivity from orbital platforms. These devices have been deployed for many years. They typically use broadband radiometers and/or a collection of narrow band measurements. Often they also include some sort of ground truth measurement to support the space-based measurement. These instruments are again unsuitable for robust man-portable operations.
Field instruments do exist, however. One class of instruments normally attempts to measure emissivity by measuring reflectance and calculating the emissivity based on this measurement. Surface emissivity is measured indirectly by assuming that ε=1−reflectivity. In general, a single energy bounce is measured and the reflected energy is measured. Typically, a large intensity laser is used as a radiation source and the strength of the reflected intensity is measured. This allows for the calculation of measured reflectance and hence resultant emissivity at the wavelength of the laser. These laser instruments can work adequately in radiation bands in which there is no emission, such as the visible band, but become problematic if thermal emission sources in the object being measured must compete with reflected laser intensities. These laser devices also suffer from the fact that they are inherently based on measurements at a single wavelength or, at best, a small number or wavelengths and they generally do not provide large field of view emissivity images of the object of interest.
Another class of field instruments for measuring emissivity contains single band radiometers. These unfortunately require knowledge of the surface temperature and again suffer from an inability to unravel reflected and emitted light from the source.
For example, U.S. Pat. No. 5,272,340 to Anbar shows an infrared imaging system for simultaneous generation of temperature, emissivity, and fluorescence images that determines temperature, reflectivity, and fluorescence of a surface. U.S. Pat. No. 5,868,496 to Spitzberg shows a method to calculate surface temperature from an object by measuring radiated energy in multiple wavelength bands. U.S. Pat. No. 4,659,234 to Brouwer et al. shows a method to correct emissivity readings for a radiation thermometer by measuring radiated energy at two wavelengths. U.S. Pat. No. 4,974,182 to Tank shows a method for measuring the emissivity and temperature of an object by successive determination of radiance in multiple wavelength bands.
Thus, although substantial effort has been devoted in the art heretofore towards development of methods to measure temperature and emissivity, there remains an unmet need for a robust device which is easier to use and which can be deployed to an operational military environment. Likewise, there remains an unmet need for a method to measure temperature and emissivity that corrects for atmospheric conditions and environmental radiance.